Some high sulfur coals and fuel oils which are burned in the boiler-generator units of electrical power stations and other industrial furnaces are the source of air pollution problems in heavily populated areas. The flue gas from these fuels contains SO.sub.2 and SO.sub.3 and efforts are being made to develop efficient processes for removing these materials. While the problem is generally greater for electric power plants, it is also a problem for other industries where heat must be generated in a boiler, and ore smelting and the like.
Economic considerations are dominating evaluation of the various candidate processes. Many processes for removing SO.sub.2 and SO.sub.3 are being developed which are satisfactory from a technical standpoint. (See Arthur M. Squires - "Air Pollution: The Control of SO.sub.2 from Power Stacks," Chemical Engineering, Nov. 20, 1967, pp. 133-140; and A. V. Slack, "Air Pollution: The Control of SO.sub.2 from Power Stacks. Part III--Process for Recovering SO.sub.2, " Chemical Engineering, Dec. 4, 1967, pp. 188-196, and British Pat. No. 1,089,716.) However, no process has been generally accepted for commercial use because of the high investment and operating costs of the known processes. Processes which provide by-product credits are obviously more attractive and one process currently being considered involves oxidizing sulfur dioxide in the flue gas to the trioxide, cooling to condense sulfuric acid formed by reaction of the trioxide with steam and recovering the acid in about 70 to 80% concentration. See Chemical Engineering, June 8, 1964, pp. 92-94. Aside from cost, the chief disadvantages of this process are that the sulfuric acid mist causes corrosion problems and that the demand for 70% sulfuric acid is not as great as the demand for more concentrated sulfuric acid such as 98-100% sulfuric acid. Furthermore, little or no nitric oxide is removed.